Fix slow specs in roadmap

*How?*
- Set up store *before* component
- Only pass a subset of epics instead of all of them

*Follow up*
Let's address the per-epic slowness in a
[follow up][1].

[1]: https://gitlab.com/gitlab-org/gitlab/-/issues/217140

ee/spec/frontend/roadmap/components/roadmap_app_spec.js

@@ -171,8 +171,8 @@ describe('RoadmapApp', () => {
     let roadmapTimelineEl;
 
     beforeEach(() => {
+      store.dispatch('receiveEpicsSuccess', { rawEpics: rawEpics.slice(0, 2) });
       wrapper = createComponent(mount);
-      store.dispatch('receiveEpicsSuccess', { rawEpics });
       roadmapTimelineEl = wrapper.find('.roadmap-timeline-section').element;
     });